Vibration device



Dec. 3, 1929. E. H. SMYTHE ET AL 1,738,327

VIBRATION DEVICE Filed March 30, 1927 2 Sheets-Sheet l [ow/N /7 SM1/THE /NvfA/ro/fs. QAM-CE E, jy

Arm/MEV Patented Dec. 3, 1929 UNITED STATES PATENT OFFICE EDWIN H.' SMYTHEf OF EVANSTON, ILLINOIS, AND CLARENCE E. LANE, 0F MONT- CLAIR, NEW JERSEY, ASSIGNORS T0 BELL TELEPHONE LABORATORIES, INCORPO- RATED, 0F NEW YORK, N. Y., A CORPORATION 0F NEW YORK VIBRATION DEVICE Application led March 30, 1927. Serial No. 179,453.

This -invention relates to vibration devices and particularly to such devices for radiating sound waves.

An object of the invention is to'simulate the transmission characteristics of a long vibration transmitting path by means of a relatively short path.

Another object is to radiate sound with substantially uniform eiiiciency over a wide frequency range.V

In accordance with the present invention, there is provided a vibration device comprising a plurality, of vibratile sections having dierent propagation constants, respectively, and having their impedances substantially matched sol as to reduce reflection of vibrations from the joined portions.- Vibratory t energy is preferably applied to the device at a section in which the velocity of wave propagation is greatest and the device is prefverably supported at a section in which the velocity of propagation is least.

In accordance with an embodiment of the invent-ion herein shown and described for the purpose of illustration, diaphragm sections having diferent propagation constants and substantially matched impedances are obtained by employing in the different sections, materials of different mass and stiffness, respectively, per unit length in the direction of propagation of vibration. The width of the diaphragm and the llexural stiffness for lengths of equaly masses are maintained substantially constant throughout the whole Alengthof the diaphragm.

This invention may be readily understood by referring to the following fdetailed description and the accompanying drawings in which:

Fig. 1 is a front elevation, partly in sect-ion, of a sound radiator constructed in accordance with the present invention;

Fig-2 is a sectional view taken along the line 2--2 of Fig. l; l

Figs. 3 and 4; s how modications of the lsound radiator of Fig. 1; and y Fig. 5 is a sectional view taken along th line 5 5 of Fig. 4.

In all of the sound radiators shown in the drawing, it is desired to simulate a long 12 of relatively heavy wood, such as mahogany, the gra-in of which runs vertically. These sections arebevelled at their joined edges and are glued or otherwise secured to the frame 13 along the unjoined edges which extend in the direction of the grain. Felt or other material 9 possessing negligible stiffness is secured to the end sections of the diaphragm in order to giveladded mass Without stiffness and at the same time it may serve as a damping agent to decrease the amplitude of resonance peaks due to vibrational Waves reflected from the edge of the diaphragm or to other causes. The ribs 14, 15

and 16 are provided for stilening in a transverse direction the central section 10 of the diaphragm so that at lower frequencies the driving force is essentially applied over the full width' of the diaphragm. A ldriving means such as the electromagnetic receiver V17, is secured to the upright members 18 of the frame and is connected to the rib 15 by means of the rod 19 which is secured to the rib by means of the thumb set screw 20.

While the theory of wave propagation in a diaphragm constructed in accordance with this invention is not definitely established,

and While the invention is independent of energy longitudinally through the diaphragm is desired only to the extent necessary to perlmit the lateral movement of a sullicient area of the diaphragm to make possible the lateral radiation of a desired amount of sound energy. The longitudinal ropagation out toward the support is somet ing which not only is ordinarily not usefully employed, but

In the diaphragm of this invention, the active radiatingl portion is made sulciently large to radiate a desired amount of energy intothe air and is provided'with a marginal terminal section for attenuating the wave energy propagated in the diaphragm beyond the active radiating portion so that no. large amount of reiected wave energy is returned A. lowand the mass per unit length in this from the marginal terminal section and sup'- port to the' active radiating portion of the diaphragm.y To meet this requirement for the marginal terminal section, the elastic l restoring force or stiness per unit length in the direction of wave propagation should` be direction should be high as compared with the corresponding constants for the active radiating portion. A marginal portion having such constants has a relatively low wave propagation velocity, and a relatively high value of wave energy attenuation. In order to avoid wave reiection from the joined area of the active radiating portion land the marginal terminating portion of the diaphragm, the massand stiffness constants in these portions are made such that their impedances are matched at their joined-portions. VThis is accomplished by making the stiffness for lengths of equal mass in the direction of Wave .propagation the same for the marginal .y and the radiating portions of the diaphravm.

In accordance with one embodimentof t is lnventlon 1t 1s 4desired to remove a considerable portion from the ends of a rectangular diaphragm-andsubstitute therefor a portion of shorter length without Valtering certain characteristics of the original diaphragm such as its fundamental resonance frequency and the wave propagation dist-ance between the lpoint of drive and the point of support. By substituting a section the mass per unit length of which is high compared with that of the removed portionv and/or the stiiness per unit lengthV of which is relatively low, it is possible to obtain a diaphragm such that v the time required for a wave to travel from the driven portion to the ends of the diaphragm is the same for the new diaphragm as for the original one having relatively longer dimensions. The impedance offered to vibrations transmitted along a diaphragm is a function of its mass and stiiness. Conseuently it is possible to approximately match t e impedance between the dissimilar portions of a diaphragm, and thus prevent reection vibrations at their joined portions, by making thestiness the same for lengths oequai masses'f" A specific method o computing the re- 'lo at 'each end of this diaphragm a new portion of length 11, thickness t1, andthe same width W as before such that the mass and iexural stifl'ness measured in the direction of the length for any portion n is the same for of the old, where n may be given any value. i

The mass M, for any element of the originaldiaphragm is KZ. ,t0D0 where K is a constant depending u'pon the length of the element chosen and the value of W, and Do is the new material as for the density. The mass for the corresponding element of the new material is KlltlDl, where D1 is its density'. Therefore we have forA matched impedances where Z1 and Z0 arezany lengths:

' KZ11D1=KZ00D0 or D 0 A zltl Likewise if we equate thel lexuralstiness of these elements, we have eenzaam,

ha los where AC is a constant depending on the widthf of the material and E1 and E0 are values of Youngs modulus of elasticity for the -ma- I terials. 0r

Suppose the middle portion of the diaphragm is made of balsa wood and is .47 cm.

thick to), the grain of the balsa wood run- A ning lengthwise of the diaphragm, and the two ends are made of mahogany with the, grain running crosswise. Measurements give us the following table of values:

`Substituting these values in (3) and (4) gives t1=.50 t., (e)

Z1=.36 Z0 and -will have a total length of 2Z1+L=29.'5

and the thickness of the mahogany ends will be only 3/32.

Another consideration is to assume that the added portion of the diaphragm is loaded with a uniform mass m per cm.2 which does not contribute toward the stiffness. We then have ZltlDl l1m= ZotoDo (7) Equation (2) still holds. Combining (2) and (7 gives v NIH and

t l En* Tribu 9) I er Suppose again we wish to simulate a balsa wood diaphragm 20X60"X3/16 and replace Z0=24 by mahogany grain running crosswise, letting m= .22. This gives us Z1=.17 lo (10) .=.233 t, 11)

The total length of the new diaphragm then is 20.1 and thickness of the mahogany end will be .044.

Fig.I 3 shows a modification' of the diawhere lphragm of Fig. 1, comprising a central section 21 of light stiff material and the sections 22, 23 and 24 between the central section and the two ends lof the supporting frame 25.

Section 22 has a greater mass and lower stiffness per unit length than section 21; section 23 has a greater mass and lower stiffness per .unit length than section 22, etc. The stiffness mated by employing short sections of lumped mass and lumped compliance, by compliance being meant the reciprocal of stiffness. As shown in Figs. 4 and 5 the centralor driven portion 40 of the diaphragm is preferably made of light, stiff material, and the end sections are composed of the lumped masses 41 and the lumped compliances 42 which may be formed in a diaphragm of uniform material throughout. As previously pointed out the stiffness for lengths having equal mass should be substantially the same for all portions of the diaphragm. 'If desired the masses 41 may be progressively increased from the edge of the central portion 2l out to its supported portion and nat the same time the compliances progressively increased, or, in other words, the stitnesses progressively decreased so as to maintain the' characteristic impedance of the loaded sections substantially constant while tapering the propagation constants of the sections.

Although the diaphragms shown in the drawing and described above are all rectangular in shape, it is apparent that the invention is also applicable to diaphragmsof1 other shapes, such as a circular diaphragm having acentral driven portion of light stiff material and an annular portion joined thereto of relatively heavy, iexible material. The invention, moreover, is not limited to plane diaphragms but may be applied to other diaphragms. Another application of the inventionis to diaphragms in which the restoring force is mainly tensional, instead of flexural such as in the case of the diaphragm heretofore described.

What is claimed is:

1. A vibration device, in which the mass increases and the stiffness simultaneously del c reases at a point on a line extending in the direction of propagation of vibration in said device.

2. A vibration device, in which one portion has greater mass and lower stiffness per unit length in the direction of propagation of vibration than another portion the characteristic impedances of said portions being substantially equal.

3. A vibration device in which one portion' has greater mass and lower stiffness per unit length in the direction of propagation of vibration than another portion, the stiffness of lengths having equal mass being substantially equal.

4. A vibration device comprising a vibratile means, supporting means therefor, and means for actuating said vibratile means at a l position remote from said supporting means,

vthe mass of said vibratile means 1ncreas1ng f and its stiffness decreasing from its driven portion to its supported portion.

5. A vibration device comprising a plurality of vibratile sections joined together, and

having different mass and stiness' respectively per unit length in a givendirection, said sections being so arranged that the mass per unit length in said direction in saidvibra- -tion device increases while the stiness simultaneously decreases.

6. A vibration device comprising a plurality of joined vibratile sections having different propagation constants, respectively, and lhaving their characteristic impedances substantially matched.

7. A vibration device comprising a plurality of joinedvibratilesections having difierent propagation constants; respectively, the

vcharacteristic-impedancelooking into any one of said sectionsfrom its unction with another section being such that negligible reflection occurs at said junction.

' 8. A vibration device comprising a plurality of vibratile sections joined together, the propagation constants ofat least two of said sections being unequal, andthe characteristic impedance looking into one of said secthe propagation constants of at least two ofk said sections being unequal and the characteristic'impedancelooking into one of said sections from junction with another being substantially equal to the characteristic impedance looking into the other of said sections from said junction, and means for -applying energy to a section in which the rate of propagation of vibration is highest.

11. .A vibration device` comprising a .plurality of joined vibratile sections having different propagation constants, respectively, 'the stiifrfess for lengths having equal masses `being substantially `equal hhroughout said vibration device.

12.' A sound radiator-comprising a driven portion of light material having high stiness in the direction of propagation of vibration therein and another portion joined to said driven portion having a higher density anda smaller stiffness in sai'd direction of propagation, the stilness for lengths of equal masses being substantially the-same for each of said portions..

v 13. A sound radiator comprising a plural-.

ity of joined vibratile sections, actuating means for applying energy to a section-oi said device and means for supporting another section thereof. the lengths of saidgsectionsl per unit length in the direction of wave propagation inthe diaphragm, the stiiness for lengths of equal mass in said direction of wave propagation being substantially constant' throughout the diaphragm.

.15. Ajsound radiator comprising a substantially plane diaphragm, supporting Vmeans for a peripheral portion of the diaphragm, and means for actuating a portion ofthe diaphragm rei-note from the supported peripheral portion, said diaphragm comprising a central section of light, sti material and a peripheral section of relatively heavy, flexible material, the stiffness for lengths of equal mass in the direction of wave-propagation in the diaphragm being substantially the same for both of said sections.

16. A sound-radiator comprising a central section of light wood joined to two relatively short end sections of relatively denser wood, the ain of said central section running lengt wise therein, and the grain in said end sections running perpendicular to the grain in said central section, means for driving said device near the mid-portion of said central section, ribs for increasing the stiiness of said central portion across the grain, means for supporting said end sections along the direction of the grain, and a material attached to a portion of each of said end sections for increasing the wei ht without appreciably increasing thesti ness thereof.

17. A sound radiator comprising a relatively large area driven portion having distributed mass andcompliance, and a relatively small area peripheral portion having concentrated mass and compliance.

18. A'soundo radiator comprising a relatively large area driven-portion having. disrtributed mass and compliance and a relatively .small area marginal portion having concentrated mass and compliance of such .values that th`e characteristic impedance of said marginal portion is substantially equal to .the characteristic impedance of said driven portion.

19. A sound radiator comprising a driven portion; having distributed, mass and compliance, a support, 'and a supporting marginal portion interposed between said driven portion .and vsaid support, said driven portion and said mar 'nalportion havin substantially equal .c aracteristic impe ances, the propagation constant of said marginal 'laA 1,vas,s27

portion being tapered from said driven portion to thev support of said vibration device.

20. A sound radiator comprising a diaphragm, certain portions of which vary in density and have different propagation constants from other portions thereof, all of said l portions .having their characteristic impedances substantially matched.

21. A sound radiator comprising a diaphragm, consisting of portions having differ- 10 ent constants for the propagation of sound, the elasticity of said diaphragm between said portions decreasing from the point of drive outwardly.

Y In Witness whereof, we hereunto subscribe 15 our names this 22 day of *March A. D., 1927.

EDWIN H. SMYTHE. CLARENCE E. LANE. 

